Misty Brooder

Like many have done, I am using a plastic tote for my brooder box.
For my litter, I have been using reptile terrarium sand.
My heater is a combination of sitting in front of the fireplace, and a reptile heat lamp.
My food/water dishes are the ones that come with the TSC chick starter kit - just regular yellow plastic things.
The chicks seem to love the sand. They scratch and take dust baths.
It's very easy to clean (I use another reptile terrarium product, a scoop that looks a little like a kitty litter scoop but more meshy).
The chicks are happy and active, and starting to get big girl feathers and getting larger at a nice rate, so all in all I think I did a pretty good job with my first brooder.
The chicks are now 2 weeks old.
I just recently moved them to a larger tote, the problem did not exist before this - but nothing has changed in their box other than size.
But here's my concern:
It's getting misty.
There is moisture on the walls of my chickerrarium.
The "cause" is definitely a combination of the heat and the water dish and all that sciencey evaporation water cycle stuff.
Is this going to harm them?
It isn't raining down the sides, but there is enough moisture that when they rub against the side, their feathers will become damp.
The damp feathers would definitely be a problem if they were cold. But the damp is caused by the heat. If I made the ventilation larger on the top it would probably reduce the misting, but that would also make heat escape...
Is this a "problem" that needs to be "fixed"?

Open the top up. Warm + damp = perfect conditions for bacteria and mold to flourish. Ick. As long as they have a warming place and draft protection, the rest of the brooder can be cool or even cold. Really.